1. Field of the Invention
The present invention relates to atomic stabilized frequency sources and more particularly to an improved housing for the lamp and associated circuitry for a rubidium frequency standard.
2. Prior Art
Although the present invention may find practical application in any one of numerous atomic stabilized frequency sources, it is particularly adaptable for operation in a rubidium vapor cell frequency standard. Rubidium vapor cell frequency standards, as well as other types of atomic stabilized frequency sources, are described extensively in the literature. For example, reference may be had to the texts respectively entitled, "Frequency and Time" by P. Kartaschoff, Academic Press, 1978; and "Frequency Synthesizers Theory and Design", Second Edition by Vadim Manassewitsch, John Wiley & Sons, 1980. Such frequency sources are stabilized by quantum mechanical atomic state transition resonances such as the hyperfine atomic resonance frequency related to the change in the internal energy of the atom. A rubidium frequency standard operates as a descriminator based upon the energy absorption characteristics of rubidium-87. In practice, a rubidium lamp passes a light beam into a rubidium absorption cell. The rubidium cell absorbs some of the light energy because of the energy level transitions in the rubidium-87 gas. When an electromagnetic field frequency equal to the resonance frequency of the rubidium vapor is applied to the vapor cell, the number of energy level transitions in the rubidium-87 gas is increased and more of the light emitted by the rubidium lamp is absorbed by the rubidium vapor cell. Typically, a photodiode is used to detect the occurrence of the maximum absorption of light from the rubidium lamp which occurs when the frequency of the excitation electromagnetic field exactly matches the rubidium resonance frequency. Typically, a frequency synthesizer is used to generate the appropriate electromagnetic field frequency of approximately 6,834.685 MHz. This field frequency is modulated at a relatively slow rate (i.e., 154 Hz.) so that the photodiode provides a demodulated signal which may be applied to a phase detector or comparator which also receives a reference modulation signal. The output of the phase comparator is a DC error voltage which is used to control a voltage variable crystal oscillator at a selected frequency, typically of 5 or 10 MHz. In this manner, the frequency of the crystal oscillator is stabilized to approximately one part in 10.sup.11 or better over long periods of time to provide a highly stable and accurate frequency source.
Ignition and subsequent controlled operation of the rubidium lamp requires both radio frequency field excitation coupling (because it is an electrodeless lamp) and good thermal coupling to a temperature controlled heater. Because the RF power required to ignite and maintain the lamp discharge is relatively high (on the order to 2 watts DC input to an RF circuit in the preferred embodiment) for a compact frequency standard, a substantial degree of RF shielding is required to avoid interference between the RF signal and other circuits of the standard.
Those familiar with the art to which the present invention pertains will appreciate the requirement for utilizing an active heater element in conjunction with the lamp in order to raise the operating temperature to a suitable point for proper ignition and discharge with the requisite degree of intensity and proper spectral components. In the embodiment of the invention disclosed herein, lamp operating temperature is 137 degrees C. This high temperature would normally place a severe strain on the active heater element, typically, a bi-polar transistor in the prior art, because of approaching the secondary breakdown limit of the transistor. Furthermore, in the prior art, the housing for a rubidium lamp typically includes a gap or slot to avoid the effect of a shorted turn with respect to the discharge igniting coil contained within the housing. As a result the use of any kind of thermal material around the housing that requires good bonding thereto would be precluded by the slot in the housing.
As a result of the above-noted disadvantages of prior art lamp housing and associated field excitation and heating techniques, it has been necessary in the past to make a variety of mechanical and electrical assembly trade-offs in lamp structure design, particularly for rubidium frequency standards of compact size. Consequently, comparable prior art devices have had to be designed to accommodate such constraints by either making the frequency standard larger and more expensive, or less reliable and durable and subject to greater maintenance costs.